Extract from St Alphonsus Liguori: Moral Theology, Bk. 6, nn. 95-7.
Concerning Baptism
Baptism, therefore, coming from a Greek word that means ablution or immersion in water, is distinguished into Baptism of water ["fluminis"], of desire ["flaminis" = wind] and of blood.
We shall speak below of Baptism of water, which was very probably instituted before the passion of Christ the Lord, when Christ was baptised by John. But Baptism of desire is perfect conversion to God by contrition or love of God above all things accompanied by an explicit or implicit desire for true Baptism of water, the place of which it takes as to the remission of guilt, but not as to the impression of the [baptismal] character or as to the removal of all debt of punishment. It is called "of wind" ["flaminis"] because it takes place by the impulse of the Holy Ghost who is called a wind ["flamen"]. Now it is de fide that men are also saved by Baptism of desire, by virtue of the Canon Apostolicam, "de pres-bytero non baptizato" and of the Council of Trent, session 6, Chapter 4 where it is said that no one can be saved "without the laver of regeneration or the desire for it".
Baptism of blood is the shedding of one's blood, i.e. death, suffered for the Faith or for some other Christian virtue. Now this Baptism is comparable to true Baptism because, like true Baptism, it remits both guilt and punishment as it were ex opere operato. I say as it were because martyrdom does not act by as strict a causality ["non ita stricte"] as the sacraments, but by a certain privilege on account of its resemblance to the passion of Christ. Hence martyrdom avails also for infants seeing that the Church venerates the Holy Innocents as true martyrs. That is why Suarez rightly teaches that the opposing view [i.e. the view that infants are not able to benefit from Baptism of blood – translator] is at least temerarious. In adults, however, acceptance of martyrdom is required, at least habitually from a supernatural motive.
It is clear that martyrdom is not a sacrament, because it is not an action instituted by Christ, and for the same reason neither was the Baptism of John a sacrament: it did not sanctify a man, but only prepared him for the coming of Christ.
St. Robert Bellarmine, Of The Church Militant, III, 3, “Of those who are not baptized
*“Martyrdom is rightly called, and is, a certain baptism.” (On the Sacrament of Baptism, Bk. I, Ch. VI, (Tom. 3, p. 120A))
“Concerning catechumens there is a greater difficulty, because they are faithful [have the faith] and can be saved if they die in this state, and yet outside the Church no one is saved, as outside the ark of Noah. […] I answer therefore that, when it is said outside the Church no one is saved, it must be understood of those who belong to her neither in actual fact nor in desire [desiderio], as theologians commonly speak on baptism. Because the catechumens are in the Church, though not in actual fact, yet at least in resolution [voto], therefore they can be saved. (Of The Church Militant, III,
3, “Of those who are not baptized”)
Douay Catechism (by Henry Tuberville, D.D. 1649)
“Q. Can a man be saved without baptism?
“A. He cannot, unless he have it either actual or in desire, with contrition, or to be baptized in his blood as the holy Innocents were, which suffered for Christ.”
Mgr. J. H. Hervé, Manuale Theologiae Dogmaticae (Vol. III: chap. IV) - 1931
II. On those for whom Baptism of water can be supplied:
"The various baptisms: from the Council of Trent itself and from the things stated, it stands firm that Baptism is necessary, yet in fact or in desire; therefore in an extraordinary case it can be supplied. Further, according to the Catholic doctrine, there are two things by which the sacrament of Baptism can be supplied, namely an act of perfect charity with the desire of Baptism and the death as martyr. Since these two are a compensation for Baptism of water, they themselves are called Baptism, too, in order that they may be comprehended with it under one as it were generic name; so the act of love with desire for Baptism is called Baptismus flaminis (Baptism of the Spirit) and the martyrium (Baptism of Blood)."
Pope Pius XII, Address to Italian Midwives
If what We have said up to now deals with the protection and the care of natural life, it should hold all the more in regard to the supernatural life which the newly born infant receives with Baptism. In the present economy there is no other way of communicating this life to the child who has not yet the use of reason. But, nevertheless, the state of grace at the moment of death is absolutely necessary for salvation. Without it, it is not possible to attain supernatural happiness, the beatific vision of God. An act of love can suffice for an adult to obtain sanctifying grace and supply for the absence of Baptism; for the unborn child or for the newly born, this way is not open.